High contrast photographic emulsions



S at Pa Y; 3,393,073 HIGH CONTRAST PHOTOGRAPHIC EMULSIONS Georges .Reutenauer, Simone Leonie Marie Elisabeth :Boyer, and Marie-Claire Andre Lucienne Prteseille, Vincennes, France, assignors'to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey No Drawing. Filed Apr. 16, 1965, Ser. No. 448,887

This invention relates to photography, and more particularlyjto high contrast gelatin silver halide emulsions and to a" method for the preparation of halftone patterns.

In the graphic arts field, it is common to provide high contrast silver halide emulsions and to expose such'emulsiolis to a hal-ftone image. Development of the exposed emulsio'n' is carried out underspecial conditions and is believed to proceed catalytically. Such development is commonly "referred to in the graphic arts field "as infectious development. Development of high contrast emulsiohs involves a primary development stage which requires a relatively long period of induction during which the latent image is very slowly developed. The primary development depends upon the amountof light received by the film. When the amountof developed silvjer" halide reaches a well defined value, infectious development occurs causing the grains to be reduced at a very high speed until'maximumdensity is reached, regardless of the amount of light received. In practice, an area which has received a uniform exposure to radiation has, upon development, some spots which are developed more rapidly than thejwhole area. It may be that these spots are more highly sensitized than the rest of the emulsion, or the developer may reach such spots more rapidly than other areas of] the film. Whatever the cause, the results are that areas which'have not received suflicient lighting to cause infectious development show black spots having a very small diameter. This is referred to herein as 'the black spots defect. This defect, in the practice of halftone photography, causes black areas between the normal dots which connect two or more dots, thus tending to result in unfavorable blocking during photo-mechanical reproduction. It is highly desirable, therefore, to provide high contrast emulsions, and a method for obtaining halftone patterns, in which the occurrence'of the black spots defect is rediiced to a minimum. f

Oine'object of our' invention is to provide high contrast photographic silver halide emulsions which have a reduced tendency to exhibit the black spots defect upon development. Another object of our invention is to provide a method for the preparation of halftone patterns which results'in 'a substantial reduction of the black spot defect. Other object of our invention will appearherein.

Iri one embodiment of our invention, we provide a high contrast'silver halide emulsion containing a high proportion of silver chloride, and a polymer selected from the group consisting of poly(N-allcanolacrylamide); poly(N- alkanol methacrylamide); and polymershaving the following forrnula; l h

In another embodiment of our invention, we provide a method'for the preparation of halftone patterns by exposing a dried, thin photographic silver halide emulsion layer one photographic support to a halftone image and develfoping the resulting exposed silver halide image layer to a high contrast, the methodof our invention featuring the improvement which comprises employing a high contrast photographic silver halide emulsion layer which contains a high percentage'of silver chloride, and ajpoly'mer selected from the group consisting of poly(N-alkanolacrylamide); poly(N-alkanolmethacrylamide); and, polymers having the following formula:

We have further found that additional reduction of the black spot defect may be achieved when the pH of the emulsion employed in the invention is lowered from its normal pH of about 4.5 to pH of about 3.5, for example, by the addition of an acid such as sulfuric acid.

The polymers which we employ in accordance with our invention may be prepared by any suitable procedure. For example, poly(N-alkanolacrylamide) may be prepared by initially forming N-hydroxyalkylacrylamide (or methacryla-mide) in accordance with the method described in U.S. Patent 2,593,888, and these monomers may be polymerized to highly water soluble polymers using a conventional polymerization catalyst such as benzoyl peroxide. Advantageously, these polymers may be prepared by reacting poly(methacrylate) or poly(methylmethacrylate) with an alkano'lamine. This method of preparing the polymers employed in the invention is described below:

Poly (N-fi-ethanolacrylamide) 10 g. poly(methacrylate) were crushed into fine particles and refluxed with stirring in cc. ethanolamine. Heating was continued until the poly(methylacrylate) was completely dissolved (about 3 to 6 hours). The mixture was cooled, and precipitated in acetone several times to remove any remaining ethanolamine. A yield of 60% poly (N-B-ethanolacrylamide) was obtained. Calculated for N: 12.1%; found: 12.0%.

The polyamide which we employ in our invention is advantageously prepared from tartaric acid and ethylenediamine. A convenient method for the preparation of this polymer is described below.

PREPARATION OF PREPOLYMER 37.5 grams of finely crushed tartaric acid were placed in a three neck flask and dissolved with stirring in a mixture of 42 parts ethyl alcohol and 7 parts water. The temperature was raised to 70 and, upon complete dissolution of the tartaric acid, the temperature was reduced to 40 C. Ethylenediamine was then poured into the solution by means of a dropping funnel. The reaction was exothermic and the reaction vessel was cooled with ice water to maintain the temperature at about 40 C. during the reaction. The product was purified by dissolving in water followed by precipitation with methyl alcohol.

PREPARATION OF POLYMER Polymerization of the prepolymer was carried out by dissolving 60 grams of the prepolymer in 600 cc. of metacresol. The mixture was mechanically stirred and heated. Polymerization began at about C. and the temperature was maintained between 170 C. and C. until all the polymer had dissolved. The polymer was precipitated from solution by pouring in a large volume of a methanol solution. The precipitate was washed by decantation in alcohol, drained, dissolved in water and reprecipitated with methyl alcohol, drained and washed with acetone to facilitate drying. The polymer is hygroscopic and preferably is obtained in an atmosphere of low relative humidity. The degree of polymerization was about 25 and the molecular weight about 5,000.

Our invention will be more fully demonstrated by the following examples.

Example I The ethylenediamine-tartatic acid polymer obtained above was added to a spectrally sensitized high contrast silver chlorobromide gelatin emulsion at a concentration of 25 grams of the polymer per mole of silver halide. The high contrast silver chlorobromide emulsion was prepared by the method describe-d in U.S. Patent 2,756,148 and contained 65 mole percent chloride and 35 mole percent bromide. Samples of the emulsion containing the polymer as well as samples which did not contain the polymer were coated on both cellulose acetate and polyester supports. The emulsion were coated at a coverage of 478 mg. of silver per square foot and 473 mg. of gelatin per square foot. Samples of each coating were exposed on an Eastman 1B sensitometer and processed for various times in Kodak Kodalith Developer. The results obtained are shown in Table 1 below.

The polymers useful in our invention are water soluble and include the polymer of tartaric acid and ethylenediamine, poly(N-alkanolacrylamides) and poly(N-alkanolmethacrylamides). The alkanol group in such polymers preferably contains from 1-4 carbon atoms, and advantageously is an N-fi-alkanol group. These polymers are most useful in emulsions at concentrations of from 10 30 grams of polymer per mole of silver salt.

As indicated heretofore, the effectiveness of the emulsions of our invention containing the defined polymer is increased with respect to avoidance of the black spot defect by lowering the pH of the emulsion to about 3.5. Any suitable acid (inorganic or organic) may be employed which does not interfere with the photographic TABLE 1 Rel. Speed at Various Development Times Dot Quality at Various Development Times Black in Minutes in Minutes Spot Su pport Rating at 1. 75 2. 25 2. 75 3. 25 1. 75 2. 25 2. 75 3. 25 2.25 Mm.

Dev. Time Control Cellulose Acetate 100 148 186 234 8 8 5 3 4 do 71 102 138 178 7 8 8 7 1 105 155 195 240 7 8 5 3 5 +Ply1ner .do 74 112 148 182 7 8 8 7 1 In the above table, the occurrence of the black spot defect is rated on a scale of 0 to with 5 indicating the greatest occurrence of black spots and 0 indicating a substantial absence of black spots. The scale is graded evenly and assigned numbers of from 0 to 5. As may be seen in the above table, the incorporation of ethylenediaminetartaric acid polymer substantially reduces the occurrence of the black spot defect.

Example II The emulsion described in Example I was coated onto a conventional film support, and exposed to a step Wedge through a magneta contact screen. The method of using such contact screens is described in How to Use the Kodak Magenta Contact Screen, Kodak pamphlet No. Q-21, 1956. The exposed coatings were developed for 2.25 minutes in Kodalith Developer. The occurrence of the black spot defect was rated, this time on a scale from 0 to 7. Again, the gradations from 0 to 7 are equal and the most pronounced black spot defect is indicated by 7 whereas the lower the number the greater the reduction indicated in the black spot defect. The emulsion, together with the polymers of the invention and various other related polymers were tested at normal and lowered pH with the results shown in Table 2.

TABLE 2 properties of the emulsion, sulfuric and trichloroacetic acids being highly useful. Advantageously, we employ sulfuric acid in a concentration of from about 1 to 1.5 cc. concentrated sulfuric acid per mole of silver.

The polymers which we employ in the invention may be added to the emulsion in any convenient manner and at any stage of the preparation of the emulsion with the one provision that the polymers employed must be added to an emulsion which is substantially free of gelatin hardener. However, a gelatin hardener may be incorporated in the emulsion after the addition of the polymers employed in the invention.

The high contrast emulsions employed in our invention include those which contain from to 100% silver chloride. Advantageously, the emulsions employed in our invention contain from 70 to 92% silver chloride, from 8 to 30 mole percent silver bromide and up to 5 mole percent silver iodide (it being understood that these emulsions always contain at least 60 mole percent silver chloride). We may also use, in accordance with the invention, the emulsions described in U.S. Patent 2,756,148 (1956) of MacWilliam, which emulsions contain from 40 to 90 mole percent silver chloride and from 10 to 60 mole percent bromide, which emulsions are prepared by initially precipitating a relatively chloride-rich precipitate which Addenda T1280;

Sensi- Control iil e (5 g Polylgcrylamide (24 g.)

6O Polyvinylpyrrolidone (10 g.) ACAN (24 g.)

Black Spots In the above table, PEA refers to poly(N-/3-ethan0lacrylamide), PTAEDA refers to the polymer of tartaric acid and ethylenediamine, and ACAN refers to copoly- (acrylic acid-acrylonitrile) as described in US. Patent 2,968,558. As may be seen in the foregoing table, the use of the polymers of the invention substantially reduce the occurrence of the black spot defect. The black spot defect is further reduced when emulsions containing the polymers employed in the invention contain sufiicient sulfuric acid to reduce the pH thereof. The table further shows that polymers closely related to those employed in the invention such as carboxyethylcellulose, polyacrylamide, polyvinylpyrrolidone and copoly(acrylic acidacrylonitrile) fail to provide any substantial reduction in the occurrence of the black spot defect.

is subsequently converted to a bromide-enriched silver chlorobromide by the addition of bromide ions during at least the last /3 of the precipitation cycle of the emulsion making procedure. Advantageously, the silver chlorobromide is precipitated at a higher temperature (50 C. to C.) than normally is used.

Although the emulsions of our invention are preferably gelatin silver halide emulsions, any other suitable colloid binder (suitable for dispersing light-sensitive silver halide) may be employed if desired. Typical useful colloid binders include all those described in US. Patent 3,039,873, col. 13, lines 37-71.

The term half-tone as used in the foregoing description and in the following claims means not only dot patterns but also the well known line patterns. That is, line screens can also be employed according to the present invention to provide the unusual effects illustrated.

The emulsions of our invention may contain any of the sensitizers, both chemical and optical, speed increasing compounds, gelatin plasticizers, gelatin hardeners and coating aids described in US. Patent 3,039,873, cols. 9-12, and the emulsions may be coated on any suitable support, including cellulose ester, polyester, polycarbonate, polyethylene coated paper, polyolefin, and the like.

The invention has been described in detail with particular reference to preferred embodiments thereof but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

We claim:

1. A high contrast silver halide emulsion selected from the group consisting of silver halide emulsions containing 60 to 100 mole percent chloride, to 40 mole percent bromide and 0 to mole percent iodide, and silver halide emulsions containing from 40 to 90 mole percent chloride and from 10 to 60 mole percent bromide prepared by first precipitating a relatively chloride rich silver halide precipitate and converting the precipitate to a bromide enriched silver chlorobromide emulsion by the addition of bromide ions, said emulsion having incorporated therein, in a quantity suflicient to effectively reduce black spots, a water-soluble polymer selected from the group consisting of poly(N-alkanolacrylamide); poly(N-alkanolmethacrylamide); and the polymers having the following structural formula:

0 HzN[- CHiOHiNH-ii-GHOH-OHOE-iiNH]..OHioHiNH wherein n is a whole number.

2. A photographic element comprising a high contrast photographic silver halide emulsion selected from the group consisting of silver halide emulsions containing 60 to 100 mole percent chloride, 0 to 40 mole percent bromide and 0 to 5 mole percent iodide, and silver halide emulsions containing from 40 to 90 mole percent chloride and from 10 to 60 mole percent bromide prepared by first precipitating a relatively chloride rich silver halide precipitate and converting the precipitate to a bromide enriched silver chlorobromide emulsion by the addition of bromide ions, said emulsion having incorporated therein, in a quantity suificient to effective reduce black spots, a polymer selected from the group consisting of poly (N-alkanolacrylamide) poly(N-alkanolmethacrylamide) and, polymers having the following formula:

3. A photographic element comprising a high contrast silver halide emulsion selected from the group consisting of silver halide emulsions containing 60 to 100 mole percent chloride, 0 to 40 mole percent bromide and 0 to 5 mole percent iodide, and silver halide emulsions containing from 40 to 90 mole percent chloride and from 10 to mole percent bromide prepared by first precipitating a relatively chloride rich silver halide precipitate and converting the precipitate to a bromide enriched silver chlorobromide emulsion by the addition of bromide ions, said emulsion having a pH of about 3.5, and having incorporated therein, -in a quantity sufficient to effectively reduce black spots, a polymer selected from the group consisting of poly(N-alkanolacrylamide); poly(N-alkanolmethacrylamide); and, polymers having the following formula:

wherein n is a whole number.

4. A photographic element comprising a high contrast silver halide emulsion selected from the group consisting of silver halide emulsions containing 60 to 100 mole percent chloride, 0 to 40 mole perecent bromide and 0 to 5 mole percent iodide, and silver halide emulsions containing from 40 to mole percent chloride and from 10 to 60 mole percent bromide prepared by first precipitating a relatively chloride rich silver halide precipitate and converting the precipitate to a bromide enriched silver chlorobromide emulsion by the addition of bromide ions, said emulsion having incorporated therein from 10 to 30 grams per mole of silver of water soluble poly(N-B- ethanolacrylamide) and about 1 to about 1.5 cc. of concentrated sulfuric acid per mole of silver.

5. A photographic element comprising a gelatin silver halide emulsion selected from the group consisting of silver halide emulsions containing 60 to mole percent chloride, 0 to 40 mole percent bromide and 0 to 5 mole percent iodide, and silver halide emulsions containing from 40 to 90 mole percent chloride and from 10 to 60 mole percent bromide prepared by first precipitating a relatively chloride rich silver halide precipitate and converting the precipitate to a bromide enriched silver chlorobromide emulsion by the addition of bromide ions, said emulsion having incorporated therein from 10 to 30 grams per mole of silver of a polymer having the following formula:

wherein n is a whole number.

References Cited UNITED STATES PATENTS 2,533,166 12/ 1950 Jones 260-89.7 2,593,888 4/1952 Jones 260-561 3,069,263 12/1962 Haas 96-114 XR J. TRAVIS BROWN, Acting Primary Examiner.

R. H. SMITH, Assistant Examiner. 

1. A HIGH CONTRAST SILVER HALIDE EMULSION SELECTED FROM THE GROUP CONSISTING OF SILVER HALIDE EMULSIONS CONTAINING 60 TO 100 MOLE PERCENT CHLORIDE, 0 TO 40 MOLE PERCENT BROMIDE AND 0 TO 5 MOLE PERCENT IODIDE, AND SILVER HALIDE EMULSIONS CONTAINING FROM 40 TO 90 MOLE PERCENT CHLORIDE AND FROM 10 TO 60 MOLE PERCENT BROMIDE PREPARED BY FIRST PRECIPITATING A RELATIVELY CHLORIDE RICH SILVER HALIDE PRECIPITATE AND CONVERTING THE PRECIPITATE TO A BROMIDE ENRICHED SILVER CHLOROBROMIDE EMULSION BY THE ADDITION OF BROMIDE IONS, SAID EMULSION HAVING INCORPORATED THEREIN, IN A QUANTITY SUFFICIENT TO EFFECTIVELY REDUCE BLACK SPOTS, A WATER-SOLUBLE POLYMER SELECTED FROM THE GROUP CONSISTING OF POLY(N-ALKANOLACRYLAMIDE);POLY(N-ALKANOLMETHACRYLAMIDE); AND THE POLYMERS HAVING THE FOLLOWING STRUCTURAL FORMULA: 